The three most common operation modes are:
1. Parallel line in service.
2. Parallel line out of service and earthed.
3. Parallel line out of service and not earthed.
Parallel line in service
This type of application is very common and applies to all normal sub-transmission
and transmission networks.
Let us analyze what happens when a fault occurs on the parallel line see figure 84.
From symmetrical components, we can derive the impedance Z at the relay point for
normal lines without mutual coupling according to equation 41.
Z
U
I I
Z Z
Z
U
I I K
ph
ph
ph
ph N
=
+ ⋅
−
⋅
=
+ ⋅
3
3
3
0
0 1
1
0
IECEQUATION1275 V2 EN (Equation 87)
Where:
Uph is phase to earth voltage at the relay point.
Iph is phase current in the faulty phase.
3I0 is earth fault current.
Z1 is positive sequence impedance.
Z0 is zero sequence impedance.
Z
0m
A B
Z< Z<
IEC09000250_1_en.vsd
IEC09000250 V1 EN
Figure 99: Class 1, parallel line in service
The equivalent circuit of the lines can be simplified, see figure 85.
1MRK 502 071-UEN - Section 8
Impedance protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 221
Application manual
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